In addition to primary nutrients, such as carbon, hydrogen, oxygen, nitrogen, phosphorous, and potash, micronutrients and secondary nutrients are elements which are also essential for plant growth, but are required in much smaller amounts than those of the primary nutrients. Secondary nutrients can include, for example, calcium (Ca), sulfur (S), and magnesium (Mg). Micronutrients can include, for example, boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), chlorine (Cl), cobalt (Co), sodium (Na), and combinations thereof.
Micronutrient sources vary considerably in their physical state, chemical reactivity, cost, and availability to plants. Four main classes of micronutrient sources include: (1) inorganic products such as oxides, carbonates, and metallic salts such as sulfates, chlorides, and nitrates; (2) synthetic chelates formed by combining a chelating agent with a metal through coordinate bonding; (3) natural organic complexes including lignosulfonates, polyflavonoids, and phenols, made by reacting metallic salts with certain organic by-products of the wood pulp industry or related industries; and (4) frits, or fritted glassy products, containing a concentration of micronutrient from about 2 to 25 percent.
The most common method of micronutrient application for crops is soil application. Recommended application rates usually are less than 10 lb/acre on an elemental basis so uniform application of micronutrient sources separately in the field can be difficult. Including micronutrients with mixed fertilizers is a convenient method of application and allows more uniform distribution with conventional application equipment. Costs also are reduced by eliminating a separate application step. Four methods of applying micronutrients with mixed fertilizers can include incorporation during manufacture, bulk blending with granular fertilizers, coating onto granular fertilizers, and mixing with fluid fertilizers.
Incorporation during manufacture is the incorporation of one or more micronutrients directly in fertilizers granules, such as NPK or phosphate fertilizer, as they are being produced. This practice allows each granule of phosphate fertilizer to have a consistent concentration of the desired micronutrient(s) and uniform distribution of the micronutrient(s) throughout the granular fertilizers. Because the phosphate granules are evenly dispersed over the growing area, the contained micronutrient(s) are as well. However, because the micronutrient source is in contact with the mixed fertilizer components under conditions of high temperature and moisture during manufacture, the rate of chemical reactions with the phosphates is increased which can reduce the plant availability of some micronutrients because the micronutrient(s) remain in the phosphate granule.
Bulk blending with granular fertilizers is the practice of bulk blending separately granulated micronutrient compounds with granular phosphate fertilizers and granular potash fertilizers. The main advantage to this practice is that fertilizer grades can be produced which will provide the recommended micronutrient rates for a given field at the usual fertilizer application rates. The main disadvantage is that segregation of nutrients can occur during the blending operation and with subsequent handling. In order to reduce or prevent size segregation during handling and transport, the micronutrient granules must be close to the same size as the phosphate and potash granules. Because the micronutrients are required in very small amounts for plant nutrition, this practice has resulted in granules of micronutrients unevenly distributed and generally too far from most of the plants to be of immediate benefit as most migrate in soil solution only a few millimeters during an entire growing season.
Coating of granular fertilizers decreases the possibility of segregation. However, some binding materials are unsatisfactory because they do not maintain the micronutrient coatings during bagging, storage, and handling, which results in segregation of the micronutrient sources from the granular fertilizer components. Steps have been taken to reduce the segregation problem in the case secondary nutrients and micronutrients, for example as in the case of sulfur or sulfur platelets in the fertilizer portion as described in U.S. Pat. No. 6,544,313 entitled “Sulfur-Containing Fertilizer Composition and Method for Preparing Same” and in the case of micronutrients as described in U.S. Pat. No. 7,497,891 entitled, “Method for Producing a Fertilizer with Micronutrients,” both of which are incorporated herein by reference in their entireties.
Similar to incorporation of micronutrients during manufacture described above, the micronutrient source is in contact with the fertilizer components in a coated product and the micronutrients can undergo chemical reactions with the phosphates, thereby reducing the plant availability of some micronutrients because the micronutrient(s) remain in the phosphate granule.
There remains a need for a fertilizer product that contains one or more micronutrients that maximizes the introduction of the micronutrient(s) into soil solution and ultimately to the root zone of plants.